1. Field of the Invention
The present invention relates to a process and apparatus for separation of hydrocarbons, used for separating and recovering hydrocarbons such as ethane, propane and the like from a liquefied natural gas.
2. Background of the Invention
It is being conducted generally to liquefy a natural gas at about −162° C. at around atmospheric pressure, send the liquefied natural gas to a marketplace by marine transportation, vaporize the liquefied natural gas, then feed it into a natural gas pipeline as a town gas or as a fuel for thermal power generation. Incidentally, a natural gas liquefied at around atmospheric pressure is called liquefied natural gas (LNG). The liquefied natural gas received at the marketplace contains, in some cases, a large amount of hydrocarbons of 2 to 5 carbon atoms. Such a liquefied natural gas has a high calorific value and therefore may not meet the natural gas specification required by the marketplace. Or, the hydrocarbons of 2 to 5 carbon atoms (e.g. ethane and propane) contained in liquefied natural gas can be used as a raw material for petrochemical plants and therefore have, in some cases, a higher commercial value than when used as a town gas or as a fuel for thermal power generation. Hence, it has been desired to separate and recover hydrocarbons of 2 or more carbon atoms from a liquefied natural gas received by the marketplace before the natural gas is fed into a natural gas pipeline.
For separation and recovery of hydrocarbons such as ethane, propane and the like from a liquefied natural gas, it is possible to apply a process described in U.S. Pat. No. 6,510,706 (Patent Literature 1). This process is concerned with a technique of liquefying, for liquid transportation, a natural gas at a temperature exceeding −112° C. (−170° F.) and sufficient for the liquefied natural gas at or below its bubble point. Incidentally, a natural gas liquefied in a pressurized state is called pressurized liquid natural gas (PLNG) and is distinguished from the above-mentioned liquefied natural gas (LNG). According to the technique, hydrocarbons less volatile than methane can be removed from a pressurized liquid natural gas (PLNG). This technique may be applied to a process for separating and recovering hydrocarbons such as ethane, propane and the like from a liquefied natural gas (LNG) transported at around atmospheric pressure at about −162° C. In the literature, a process is described including heating feed PLNG in a heat exchanger, thereby vaporizing at least a portion of the PLNG; passing the partially vaporized PLNG to a fractionation column; withdrawing a liquid stream enriched with hydrocarbons less volatile than methane from a lower portion of the fractionation column; withdrawing a vapor stream from an upper portion of the fractionation column; and passing the vapor stream to the heat exchanger to condense the vapor to produce PLNG lean in hydrocarbons less volatile than methane. In the literature, it is also described that when higher recovery rate of ethane and propane is desired, a reflux effect is obtained by withdrawing part of the feed PLNG and feeding it into the top of the distillation column in a liquid state without the vaporization by the heat exchanger, whereby ethane and propane can be obtained at a higher recovery rate.
For separation and recovery of hydrocarbons such as ethane, propane and the like from a liquefied natural gas, there can also be used a process disclosed in U.S. Pat. No. 2,952,984 (Patent Literature 2). In the literature, there is described a process including feeding a liquefied natural gas into the middle portion of a fractionation column; heating the contents of the lower portion of the fractionation column to produce methane-enriched vapors in the upper portion of the fractionation column; withdrawing vapors from the upper portion of the fractionation column and directly passing the vapors in heat exchange relation with the liquefied natural gas being fed to the fractionation column, to heat the feed and cool the vapors; separating condensed liquid from the vapors; refluxing the condensed liquid to the upper portion of the fractionation column; and withdrawing a heavier hydrocarbon from the lower portion of the fractionation column.
When the process described in the Patent Literature 1 is applied for separation and recovery of hydrocarbons such as ethane and the like from a liquefied natural gas, no high reflux effect is obtained because a feed liquefied natural gas which is low in methane concentration is used as a reflux for distillation column, and the recovery rate of ethane is considered to remain at about 92%. Here, the ethane recovery rate means a proportion at which the ethane contained in feed liquefied natural gas is separated from a product liquefied natural gas and recovered as a component of NGL(natural gas liquid); that is, the ethane recovery rate is a value obtained by dividing the ethane amount in NGL by the ethane amount in feed liquefied natural gas. Such an ethane recovery rate, i.e., at most about 92%, may be sufficient in order to adapt the product LNG to the natural gas specification of the marketplace; however, in order to obtain hydrocarbons of 2 to 5 carbon atoms as a raw material gas for a petrochemical plant, recovery of a higher amount of ethane for effective ethane utilization is desired from an economical standpoint. Thus, further improvement in ethane or propane recovery rates has been desired.
In the process described in the Patent Literature 2, the condensed liquid of an overhead gas of distillation column in which methane is concentrated, is fed as a reflux for the distillation column; therefore, there is an advantage of high refluxing effect. In this process, the gas from the upper portion of the distillation column is cooled, the resulting condensed liquid is separated, a residue natural gas is withdrawn in a gaseous state and then compressed by a compressor to a pressure required for pipeline transportation. Therefore, there has been a problem in that a large energy is required for the gas compression.